In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer containing a substance with a light-emitting property is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. Besides, such a light-emitting element has advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained. Thus, a large-area element utilizing planar light emission can be formed. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to lighting and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. When an organic compound is used as a light-emitting substance, by voltage application to a light-emitting element, electrons and holes are injected into a layer including the light-emitting organic compound from a pair of electrodes, whereby current flows. Light is emitted when the carriers (electrons and holes) are recombined and the electrons and holes of the organic compound returns to the ground state from the excited state where both the electrons and the holes are generated in organic molecules with a light-emitting property.
Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element. It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In addition to light emission by recombination of current excitation carriers, which is described above, there is a method in which excitation energy is transferred to another organic compound, whereby the organic compound is excited to provide light emission. This is an element structure in which a light-emitting material is diffused (doped) to the light-emitting layer in general organic EL. A host means a material into which a light-emitting material is diffused and a dopant means a material which is diffused into the host. This, in order to solve a problem in that organic molecules to provide light emission have low light emission efficiency because stacking interaction occurs when they are high concentration (concentration quenching), contributes to higher light emission efficiency by doping the organic molecules to the host and suppressing stack. At this time, the excitation energy by current excitation is transferred to the dopant from the host excited by current excitation, so that the dopant emits light.
This excitation energy transfer occurs only when transfer from high excitation energy to low excitation energy is performed. Therefore, a material having a high excitation state is preferably used for a host material.
An organic EL layer has a plurality of layers, and a carrier-transport layer is generally provided between a light-emitting layer and an electrode. As one of the reasons, a carrier-transport layer can prevent excitation energy in the light-emitting layer from quenching caused by energy transfer to the electrode. Further, a material (an exciton-blocking material) having higher excitation energy than a light-emitting layer is preferably used for a carrier-transport layer which is adjacent to the light-emitting layer so that excitation energy in the light-emitting layer is not transferred.
As another reason to provide a carrier-injection layer and a carrier-transport layer between a light-emitting layer and an electrode in an organic EL, it is to adjust a carrier injection partition between adjacent layers. Accordingly, recombination can be efficiently performed in the light-emitting layer.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out (for example, see Patent Document 1).